1. Technical Field
This invention relates to the field of medical diagnostic imaging and more particularly to an improved x-ray detector for use in digital radiography and fluoroscopy. The detector provides separate simultaneous representations of different energy radiation emergent from a subject.
2. Background Art
Radiography and fluoroscopy are long well known diagnostic imaging techniques.
In a conventional radiography system, an x-ray source is actuated to direct a divergent area beam of x-rays through a patient. A cassette containing an x-ray sensitive phosphor screen and film is positioned in the x-ray path on the side of the patient opposite the source. Radiation passing through the patient""s body is attenuated in varying degrees in accordance with the various types of tissue through which the x-rays pass. The attenuated x-rays from the patient emerge in a pattern, and strike the phosphor screen, which in turn exposes the film. The x-ray film is processed to yield a visible image which can be interpreted by a radiologist as defining internal body structure and/or condition of the patient.
In conventional fluoroscopy, a continuous or rapidly pulsed area beam of x-rays is directed through the patient""s body. An image intensifier tube is positioned in the path of the beam opposite the source with respect to the patient. The image intensifier tube receives the emergent radiation pattern from the patient, and converts it to a small, brightened visible image at an output face. Either a mirror or closed circuit television system views the output face and produces a dynamic real time visual image, such as on a CRT, a visual image for interpretation by a radiologist.
More recently, digital radiography and fluoroscopy techniques have been developed. In digital radiography, the source directs x-radiation through a patient""s body to a detector in the beam path beyond the patient. The detector, by use of appropriate sensor means, responds to incident radiation to produce analog signals representing the sensed radiation image, which signals are converted to digital information and fed to a digital data processing unit. The data processing unit records, and/or processes and enhances the digital data. A display unit responds to the appropriate digital data representing the image to convert the digital information back into analog form and produce a visual display of the patient""s internal body structure derived from the acquired image pattern of radiation emergent from the patient""s body. The display system can be coupled directly to the digital data processing unit for substantially real time imaging, or can be fed stored digital data from digital storage means such as tapes or discs representing patient images from earlier studies.
Digital radiography includes radiographic techniques in which a thin fan beam of x-ray is used, and other techniques in which a more widely dispersed so-called xe2x80x9carea beamxe2x80x9d is used. In the former technique, often called xe2x80x9cscan (or slit) projection radiographyxe2x80x9d (SPR) a fan beam of x-ray is directed through a patient""s body. The fan is scanned across to the patient, or the patient is movably interposed between the fan beam x-ray source and an array of individual cellular detector segments which are aligned along an arcuate or linear path. Relative movement is effected between the source-detector arrangement and the patient""s body, keeping the detector aligned with the beam, such that a large area of the patient""s body is scanned by the fan beam of x-rays. Each of the detector segments produces analog signals indicating characteristics of the received x-rays.
These analog signals are digitized and fed to a data processing unit which operates on the data in a predetermined fashion to actuate display apparatus to produce a display image representing the internal structure and/or condition of the patient""s body.
In use of the xe2x80x9careaxe2x80x9d beam, a divergent beam of x-ray is directed through the patient""s body toward the input face of an image intensifier tube positioned opposite the patient with respect to the source. The tube output face is viewed by a television camera. The camera video signal is digitized, fed to a data processing unit, and subsequently converted to a tangible representation of the patient""s internal body structure or condition.
One of the advantages of digital radiography and fluoroscopy is that the digital image information generated from the emergent radiation pattern incident on the detector can be processed, more easily than analog data, in various ways to enhance certain aspects of the image, to make the image more readily intelligible and to display a wider range of anatomical attenuation differences.
An important technique for enhancing a digitally represented image is called xe2x80x9csubtractionxe2x80x9d. There are two types of subtraction techniques, one being xe2x80x9ctemporalxe2x80x9d substraction, the other xe2x80x9cenergyxe2x80x9d subtraction.
Temporal, sometimes called xe2x80x9cmask modexe2x80x9d subtraction, is a technique that can be used to remove overlying and underlying structures from an image when the object of interest is enhanced by a radiopaque contrast agent, administered intra-arterially or intra-venously. Images are acquired with and without the contrast agent present and the data representing the former image is subtracted from the data representing the latter, substantially cancelling out all but the blood vessels or anatomical regions containing the contrast agent. Temporal subtraction is, theoretically, the optimum way to image the enhancement caused by an administered contrast agent. It xe2x80x9cpullsxe2x80x9d the affected regions out of an interfering background.
A principle limitation of digital temporal subtraction is the susceptibility to misregistration, or xe2x80x9cmotionxe2x80x9d artifacts caused by patient movement between the acquisition of the images with and without the contrast agent.
Another disadvantage of temporal subtraction is that it requires the use of a contrast material and changes in the contrast caused by the agent must occur rapidly, to minimize the occurrence of motion caused artifacts by reducing the time between the first and second exposure acquisition. Temporal subtraction is also not useful in studies involving rapidly moving organs such as the heart. Also, the administration of contrast agents is contraindicated in some patients.
An alternative to temporal subtraction, which is less susceptible to motion artifacts, is energy subtraction Whereas temporal subtraction depends on changes in the contrast distribution with time, energy subtraction exploits energy-related differences in attenuation properties of various types of tissue, such as soft tissue and bone.
It is known that different tissues, such as soft tissue (which is mostly water) and bone, exhibit different characteristics in their capabilities to attenuate x-radiation of differing energy levels.
It is also known that the capability of soft tissue to attenuate x-radiation is less dependent on the x-ray""s energy level than is the capability of bone to attenuate x-rays. Soft tissue shows less change in attenuation capability with respect to energy than does bone.
This phenomenon enables performance of energy subtraction. In practicing that technique, pulses of x-rays having alternating higher and lower energy levels are directed through the patient""s body. When a lower energy pulse is so generated, the detector and associated digital processing unit cooperate to acquire and store a set of digital data representing the image produced in response to the lower energy pulse. A very short time later, when the higher energy pulse is produced, the detector and digital processing unit again similarly co-operate to acquire and store a set of digital information representing the image produced by the higher energy pulse. The values obtained representing the lower energy image are then subtracted from the values representing the higher energy image.
Since the attenuation of the lower energy x-rays by the soft tissue in the body is approximately the same as soft tissue attenuation of the higher energy x-rays, subtraction of the lower energy image data from the higher energy image data approximately cancels out the information describing the configuration of the soft tissue. When this information has been so cancelled, substantially all that remains in the image is the representation of bone. In this manner, the contrast and visibility of the bone is substantially enhanced by energy subtraction.
Energy subtraction has the advantage, relative to temporal subtraction, of being substantially not subject to motion artifacts resulting from the patient""s movement between exposures. The time separating the lower and higher energy image acquisitions is quite short, often less than one sixtieth of a second.
Details of energy subtraction techniques in digital radiography and fluoroscopy are set forth in the following technical publications, all of which are hereby incorporated specifically by reference:
Hall, A.L. et al: xe2x80x9cExperimental System for Dual Energy Scanned Projection Radiologyxe2x80x9d. Digital Radiography proc. of the SPIE 314: 155-159, 1981;
Summer, F.G. et al: xe2x80x9cAbdominal Dual Energy Imagingxe2x80x9d Digital Radiography proc. SPIE 314: 172-174, 1981;
Blank, N. et al: xe2x80x9cDual Energy Radiography: a Preliminary Studyxe2x80x9d. Digital Radiography proc. SPIE 314: 181-182, 1981; and
Lehman, L.A. et al: xe2x80x9cGeneralized Image Combinations in Dual kVp Digital Radiographyxe2x80x9d, Medical Physics 8: 659-667, 1981.
Dual energy subtraction has been accomplished, as noted above, by pulsing an x-ray source in a digital scanning slit device at two kVp""s, typically 120 and 80 kVp, and sychronizing the pulses with a rotating filter which hardens the high kVp pulses by filtering out the lower energy x-ray. This results in the patient and x-ray detector sequentially seeing high energy and low energy beams from which the mass per unit area of bone and soft tissue can be solved for.
In energy subtraction, it is desirable that the two energy levels should be widely separated. This is necessary in order to accurately define the masses per unit area of bone and soft tissue.
With a slit scanning device, such as described above, sequentially pulsing the x-ray tube at 120 and 80 kVp is technically difficult and gives rise to very difficult problems in a practical clinical device. The switching frequency has to be on the order of 500 Hz. and insufficient photons (x-ray energy per pulse) results when the highest capacity x-ray tubes are combined with realistically narrow slit widths and scanning times.
In connection with CT (computerized tomography) applications, a two layer energy sensitive detector has been proposed. In this proposal, a first calcium fluoride layer is provided for sensing lower level x-ray radiation, and a second downstream sodium iodide layer senses higher energy radiation passing through the first layer. Light caused by radiation in each of the two layers is separately sensed by respective photomultiplier tubes.
The disadvantages and problems of the prior art are alleviated or eliminated by the use of an energy discriminating radiation detector including three elements. The detector includes a first element predominantly responsive to radiation of a first energy range, and a second element positioned behind the first, responsive to radiation in a second and higher energy range, along with a radiation filter interposed between the first and second elements.
Thus, an energy sensitive x-ray detector system for use in digital radiography is provided. For each picture element of the radiographic projection, the detector provides two readings from which the mass per unit area of bone and soft tissue through which the x-ray beam passes can be determined.
The energy sensitive x-ray detector employs a low atomic number phosphor screen or discrete array of phosphor segments coupled to a photodiode array, followed by a high atomic number of phosphor screen or discrete segment array similarly coupled.
An energy sensitive segment of an element of the detector system consists of a low atomic number phosphor coating layer coupled to a first photodiode, followed by a high atomic number phosphor coating layer coupled to a second photodiode. The low atomic number phosphor preferentially absorbs the low energy photons emerging from the patient and transmits most of the higher energy photons, a larger percentage of which are absorbed in the second (higher atomic number) phosphor.
Placing an appropriate filter between the two phosphor/photodiode arrays increases or hardens the effective energy of the x-ray spectrum incident on the second phosphor and results in a greater and more desirable energy separation between the x-ray spectra absorbed in the two phosphor layers.
In accordance with another embodiment, a split energy radiation detector is provided including a first energy responsive element comprising a quantity of phosphor material including one of yttrium oxysulfide and zinc cadmium sulfide, and a second energy responsive element positioned to receive energy passing through said first element, said second element including one of gadolinium oxysulfide and cadmium tungstate.
In accordance with another specific aspect of the invention, the radiation filter interposed between the two elements or layers is made of a material containing copper.
In accordance with a broader aspect of the invention, there is provided a split energy radiation detector screen comprising a deck of separate detector elements at least partially mutually superposed, each element being capable of producing information spatially locating radiation incident on the screen.
These and other aspects of the present invention will become more apparent from a consideration of the following description and of the drawings, in which: